When two asynchronous devices are connected together for the first time, it is very seldom to have a good data transmission because the asynchronous configuration of both devices are different, especially when there is no clocking signal. The four parameters of the asynchronous configuration that are to be adapted are the followings: the transmission speed, the data length, the parity if existing (even or odd), and the number of stop bits. Many cases exist depending on the combination of these parameters.
Up to now, it is necessary to know in advance the parameters of the configuration before starting the communication. The access to the documentation of the devices is difficult and sometimes impossible. Moreover, before accessing the device, it is necessary to break it down and to set the hardware parameters (jumpers etc. . . .). Therefore, there is a desire to facilitate the configuration of the parameters by implementing an adapter which can learn the configuration by reading the incoming data and which automatically set the correct parameters of the asynchronous communication link.
An asynchronous transmission is characterized by two leads, a transmit and receive lead (no clock lead). The structure of this communication is the following. When no communication occurs, a mark (high level) is on the line. This state is an idle state. When an information is to be transmitted, a start bit (low level) is sent and it synchronizes the beginning of the communication. Then, the transmission of the data bytes begins by starting to send the LSB (least significant bit). This byte is of several data bits such as 7 or 8 data bits length (the case of 9 bits length is seldom). The LSB is followed by the MSB (most significant bit). At the end of the MSB transmission, a parity bit is optionally inserted, it may be odd or even. One or two stop bits may also be sent after this optional parity bits which represent the end of the transmission. Once the transmission is completed, the line is set to idle state with a mark (high level).
The four parameters are then to be detected by an asynchronous adapter in order to establish the communication between two asynchronous devices.